Fig. 1 |. Primary metastatic profiles of leukaemia metastasis.

Acute lymphoblastic leukaemia (ALL) cells are typically initially found in the peripheral vascular system and the haematopoietic organs, including the bone marrow (BM)¹⁵, lymphatic system and spleen; however, central nervous system (CNS) involvement occurs in 5–10% of adult patients at diagnosis²³⁰ and can occur in up to 30–50% of patients in the absence of CNS-directed therapy. WHO classifies ALL and lymphoblastic lymphoma as a spectrum disorder and distinguishes these diseases according to clinical presentation favouring BM (involvement of >20% of blasts) or lymphatic tissue involvement, respectively¹⁵. Acute myeloid leukaemia (AML) primarily develops and spreads within the BM and less frequently involves other haematopoietic organs. It may invade non-haematopoietic organs including the gingiva, skin, muscle and CNS, although uncommonly. Chronic lymphocytic leukaemia/small lymphocytic lymphoma (CLL/SLL) is notable for its highly stereotyped clinical evolution: at the earliest stages, bloodstream lymphocytosis is the sole clinical finding, followed over time by the development of lymph node enlargement, then splenomegaly and in advanced stages, progressive BM involvement²³¹. CLL and SLL are considered the same entity, but with differing disease presentation of bloodstream (CLL) or nodal involvement (SLL)^232,233. Chronic myeloid leukaemia (CML) primarily involves the BM, peripheral circulation and spleen²³⁴. In rare cases including during evolution to blast crisis, CML expansion can result in infiltration into other less common sites, including the lymph nodes, liver, skin or CNS^235–237. T-ALL, T cell ALL.